The invention relates to a method of producing cold rolled, silicon-alloyed electric sheets, having a watt loss or iron loss at 10,000 gauss and 50 Hz, V.sub.10/50, of less than 2.3 watt/kp (watts per kilopound), from an unkilled mild steel melt containing 0.03 to 0.06% by weight of C. The melt is vacuum decarburized to reduce its carbon content to less than 0.02% by weight. First aluminum and subsequently silicon are added to the melt for deoxidation, whereupon the melt is cast to form slabs. The slabs are hot rolled and then cold rolled. A decarburizing and recrystallizing annealing is finally carried out in a continuous furnace for the purpose of further reducing the carbon content to less than 0.005% by weight.
It is known to start the production of electric sheets from a steel having a low content of impurities; i.e. the contents of C, Mn, S, P, Cr and Cu should be as low as possible because the watt loss or the coercive force is increased by disturbed spots of critical size, e.g. by fine oxides or by stress fields of displacements. Furthermore, it is desirable to achieve a structure in the steel as coarse-grained as possible, since this has a favorable influence on the magnetic properties.
Normally the carbon content is reduced to less than 0.005%, on the one hand, by vacuum treatment of a steel which in its original state is unkilled and, on the other hand, by decarburizing annealing of the cold rolled sheets or strips in a continuous furnace in which the recrystallization of the structure also occurs. This process is described e.g. in the German Auslegeschrift No. 1,458,852 and in the Austrian Patent Specification No. 307,469. It is desirable to keep the annealing periods in the continuous furnace as low as possible, i.e. to run the strips through the furnace at the highest possible velocities consequently, it is necessary to get the carbon content of the liquid melt to low values.
In the customary technology it has turned out that in the vacuum treatment of the molten, unkilled steel not only does a decarburization occur, but also there is a considerable reduction of the Mn-content as a consequence of the higher vapor pressure of the Mn. In steels containing less than 0.15% by weight of Mn, which content is in general considered the upper permissible limit for good qualities of electric sheets, and a carbon content of less than 0.020% by weight, in particular less than 0.010% by weight, at the end of the decarburizing vacuum treatment an unfavorable recrystallization behavior of the cold rolled sheets produced from such steels has been found. The grain was relatively fine and unduly high watt losses were measured. Attempts to increase the Mn-content brought about only minor improvements.